This invention relates to latex compositions or paint vehicles as they are sometimes referred for paint compositions. Latex paint compositions have captured a significant portion of the indoor and outdoor paint market because they have several advantages as compared with those of the organic solvent type. Three significant advantages are: the paints offer a easy mechanism for cleanup, there is substantially no air pollution and there is a reduced possibility of fire hazard. On the other hand, the coating properties and storage stability of the latex paints have been somewhat inferior to those of the solvent type, particularly in obtaining desired film thickness, durability and adhesion.
In today's environment solvent emission is of great concern and there is significant government regulation in an effort to decrease the level of volatile organic compounds (VOC) emitted to the atmosphere and to the level of human exposure concerns. Recent focus of exposure concerns has been on institutional settings where a facility is operation continuously, such as hospitals. Such an operation does not allow for painting to be conducted during "off" hours, when human exposure can be minimized. Latex formulations contribute to reduced emission and exposure.
Two types of emulsions commonly used in formulating latex paints include the all acrylic system, e.g., the systems employing copolymerized methyl methacrylate, butyl acrylate, or 2-ethylhexylacrylate with small proportions of acrylic acid, etc., as may be desired, and vinyl acetate formulations usually in combination with a small proportion of the above lower alkyl acrylates, e.g., butyl acrylate. Heretofore, the all acrylic system has been used in premium quality paints as the emulsions have provided for good water resistance, desired leveling, film hardness, durability, scrubability, etc. The vinyl acetate-acrylic copolymer systems have been utilized in formulating interior flat and semi-gloss paints and exterior house paints. The vinyl acetate-butyl acrylate latices when used in paint formulations result in paint films which have excellent toughness, scrub resistance and durability, while the vinyl acetate-dibutyl maleate emulsions impart good abrasion resistance and flexibility as well as durability.
Conventional vinyl acetate based latex vehicles often require a coalescing solvents in order for the latex to be suitable for use in a paint formulation. Coalescing solvents are incorporated into the paint composition to externally and temporarily plasticize the latex polymer for a time sufficient to develop film formation. They diffuse out of the coalesced film after film formation and thus contribute to the VOC level emitted to the environment. Paints formulated with standard vinyl-acrylic latex vehicles and no coalescing solvent generally do not pass the requirement of film formation at temperatures as low as 40.degree. F. Such paints also display cracking upon drying and provide poor durability. One approach to attempt to overcome these deficiencies is to increase the acrylate level to lower the minimum film formation temperature of the latex. Although the approach addresses the cracking and durability problems, paints prepared from these latexes are more costly and they also display dirt pickup, due to unacceptably tackiness after drying. Therefore, the hurdle is to prepare a latex which permits a low film formation temperature without causing the dried film to become tacky and to have sufficient hardness to retain good durability.
Specific examples of acrylic and vinyl ester paint vehicle systems are shown in the following U.S. Patents.
There are numerous patents which describe paint compositions incorporating a vinyl acetate-ethylene emulsion as a binder. Representative patents include U.S. Pat. Nos. 3,404,112 and 3,404,113. The '112 patent discloses the use of vinyl acetate-ethylene latexes as a film-forming binder in an aqueous paint composition. The '113 patent incorporates a triallylcyanurate in the polymerization process to enhance the degree of insolubles. The particle size of the binder will range from about 0.1 to 2 microns. The ethylene content will range generally from 5 to 40, preferably about 15% by weight of the polymer.
U.S. Pat. No. 3,440,119 discloses aqueous paint compositions incorporating an inter polymer of vinyl acetate, ethylene and glycidyl acrylate. The addition of the glycidyl acrylate into the polymer system enhances adhesion to raw wood without the use of a primer coat.
U.S. Pat. No. 4,219,454 discloses the use of a vinyl acetate copolymer emulsion for preparing semi-gloss and flat interior paint compositions. Vinyl acetate-ethylene emulsions were disclosed with preferred latexes having a particle size such that less than 5% of the particles had a size greater than 0.65 microns and less than 5% had a particle size of less than 0.33 microns. Emulsions were prepared by introducing the monomers of vinyl acetate, optionally with a small amount of butyl acrylate into a stabilizer system of water, hydroxyethyl cellulose and multiple nonionic surfactants. The Tg of the vinyl acetate ethylene polymer was approximately 22%, preferred levels of ethylene from about 10 to 15% by weight.
U.S. Pat. No. 5,470,906 discloses an aqueous ambient dry paint coating incorporating an emulsion copolymerized addition polymer containing an oligomer selected from polyurethane or polyester having a Tg below about -20.degree. C. and a number average molecular weight between 300 and 5,000. The coating is free of organic coalescing solvents. Vinyl acetate and butylacrylate (80/20) are disclosed as conventional polymeric binders for consumer based paints with the binder having an elevated Tg lowered temporarily through the use of a volatile coalescing solvent. Low molecular weight oligomers of urethanes and polyester urethane copolymers were used in place of conventional coalescing solvents to achieve desired properties without objectionable odor and VOCS.
U.S. Pat. No. 3,969,296 discloses a process for producing a vinyl acetate emulsion having improved adhesion characteristics against usual wet-cleaning with a cloth, sponge, etc. The emulsion is prepared by copolymerizing a small amount of a glycidyl ester of an alpha-beta ethylenically unsaturated acid with vinyl acetate followed by neutralization with ammonia.
U.S. Pat. No. 3,563,944 discloses a colloid-free vinyl acetate emulsion suited for producing paint formulations having good scrub resistance, film forming properties, mechanical stability, etc. The copolymer consists of vinyl acetate and a lower alkyl acrylate, or an alkyl maleate. Enhanced stability is imparted by polymerizing a portion of the monomers in a colloid-free aqueous medium and then adding more monomer during the course of reaction and using a nonionic surfactant to stabilize the polymerization.